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authorTobin C. Harding <me@tobin.cc>2017-12-20 08:17:17 +1100
committerJonathan Corbet <corbet@lwn.net>2017-12-21 13:39:27 -0700
commit227d1a61ed87033285d187214f305c27895176f9 (patch)
tree023f89073768511726dafaebb54dbeb28944dfde /Documentation/security/self-protection.rst
parentda271403a894f1139b3a49fca8fa19585902890e (diff)
doc: add documentation on printing kernel addresses
Hashing addresses printed with printk specifier %p was implemented recently. During development a number of issues were raised regarding leaking kernel addresses to userspace. Other documentation was updated but security/self-protection missed out. Add self-protection documentation regarding printing kernel addresses. Signed-off-by: Tobin C. Harding <me@tobin.cc> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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diff --git a/Documentation/security/self-protection.rst b/Documentation/security/self-protection.rst
index 60c8bd8b77bf..0f53826c78b9 100644
--- a/Documentation/security/self-protection.rst
+++ b/Documentation/security/self-protection.rst
@@ -270,6 +270,21 @@ attacks, it is important to defend against exposure of both kernel memory
addresses and kernel memory contents (since they may contain kernel
addresses or other sensitive things like canary values).
+Kernel addresses
+----------------
+
+Printing kernel addresses to userspace leaks sensitive information about
+the kernel memory layout. Care should be exercised when using any printk
+specifier that prints the raw address, currently %px, %p[ad], (and %p[sSb]
+in certain circumstances [*]). Any file written to using one of these
+specifiers should be readable only by privileged processes.
+
+Kernels 4.14 and older printed the raw address using %p. As of 4.15-rc1
+addresses printed with the specifier %p are hashed before printing.
+
+[*] If KALLSYMS is enabled and symbol lookup fails, the raw address is
+printed. If KALLSYMS is not enabled the raw address is printed.
+
Unique identifiers
------------------